A 31-year-old female rhinoceros died Friday morning at the Denver Zoo after a months-long battle with chronic renal disease.

Denver Zoo staff euthanized Shy Anne after zookeepers and veterinarians observed her condition take a turn for the worse over the past few weeks, the zoo said in a news release.

Zookeepers believed she was the sixth-oldest rhinoceros in North America.

“We knew she was not going to get better,” zoo veterinarian Diana Boon said in the release. “It’s always difficult to make these decisions, but we wanted to do what was in Shy Anne’s best interest. She will be so missed.”

Zookeepers remember Shy Anne as a feisty and determined rhino, the release said.

“This is very hard for all of our staff,” said Dale Leeds, curator of the zoo’s elephant exhibit. “We’ll always remember her as a spunky animal and full of life.”

The zoo said Shy Anne lived nearly twice the median life expectancy for a black rhino in a North American zoo, which, according to the Association of Zoos and Aquariums, is 17.8 years.

The rhino spent the last 13 years of her life in Denver. She was born at the San Antonio Zoo on Dec. 11, 1982.

Her one calf in her life, a male named Boo, was born before she was moved to Denver.

The International Union for Conservation of Nature and Natural Resources has classified the black rhino, Diceros bicornis, as critically endangered since 1996. The number of rhinos has climbed from 2,410 in 1995 to 4,880 in 2010, according to the international union. Most live in southern Africa.

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